Roughly how many loci do serious memory athletes maintain?

Hundreds to thousands, organised across many catalogued palaces.

Competitive disciplines demand huge capacity — memorising thousands of digits or many decks in an hour — so athletes build and maintain large libraries of palaces, each with dozens of loci, all rehearsed and labelled. Crucially they built up to it from small palaces; the number is the product of years of practice, not a starting requirement. For everyday use, a handful of solid palaces is plenty.